Phosphates, particularly aryl phosphates, are useful materials. These compounds do not burn easily and, in fact, many are self-extinguishing. Aryl phosphates are widely used as additives in polymeric materials as flame retardants and as plasticizers.
Carboxyaryl phosphates, i.e., an aryl phosphate wherein at least one aryl moiety is substituted with a carboxylic acid group, are also useful materials. For example, U.S. Pat. No. 3,666,840 to Ito et al. describes a class of polycarboxylic triphenyl phosphates and teaches that these materials have a number of desirable properties such as high melting points and heat stability. They are also described as being non-flammable and self-extinguishing. Because of these properties, it is taught by Ito et al. that carboxyaryl phosphates are particularly suitable as constituents of synthetic resins, synthetic fibers and other high polymer substances which are required to be heat resistant and fire resistant, and as various processing agents for imparting fire resistance or flame resistance to common synthetic resins, and natural and synthetic fibers. These phosphates are also a convenient source of various hydroxyphenyl carboxylic acids since the phosphate ester bond may be hydrolyzed to release a hydroxyphenyl carboxylic acid.
The Ito et al. patent also describes a process for preparing polycarboxytriphenyl phosphates by oxidizing the corresponding polymethyl triphenyl phosphates in a liquid phase oxidation reaction using an oxygen-containing gas and a catalyst containing vanadium, chromium, manganese, cobalt, nickel, copper or molybdenum. Cocatalysts and/or promoters such as halides, organic carbonyl compounds, peroxides and ozone are also taught as being advantageous if used in combination with the metal catalysts. The process disclosed in Ito et al., however, is impractical and would not be desirable, for example, in a commercial-scale operation. In all of the examples provided therein the reaction time for oxidizing the disclosed polymethyl triphenyl phosphates is on the order of 3 to 13 hours, and, furthermore, the specification states that reaction conditions should be chosen so that the reaction is substantially complete within a period ranging from 4 to 24 hours, excluding the induction period. These reaction times are generally too long to be useful for a commercial-scale manufacturing operation.
An improved process for preparing carboxyaryl phosphates, a process suitable for a commercial-scale manufacturing operation, would be desirable. The present invention provides such an improved process.